1. Field of Invention
This invention relates to the construction of the walls and hearths of a refractory-lined multiple hearth furnace. More particularly, the invention relates to a furnace having hearths, insulation and refractory brickwork apparatus so arranged as to avoid problems caused by expansion of the inner brickwork and differential expansion of the inner and outer walls of the furnace due to heating and cooling cycles in the furnace.
2. Information Disclosure Statement
Refractory lined multiple hearth furnaces used for incineration of municipal sewage sludge, refuse and industrial wastes, and regeneration of granular activated carbon, are well known. These furnaces commonly contain between four and twelve hearths. The typical wall and hearth construction of such a prior art furnace is as illustrated in FIG. 1. A vertically oriented cylindrical steel shell 1, comprises the exterior wall. The shell is lined with one or more layers of a low strength, high insulating capacity material 2. The interior wall is built of firebrick 3, to resist the high temperatures, and corrosive and abrasive conditions within the furnace.
Wherever a hearth is required, the steel wall is reinforced externally by a steel ring 4, known commonly as a "buckstay band". The commonly used method of making up the circumferential joint in the buckstay band is to set a splice plate across the joint and connect the plate to the band on either side of the joint with structural bolts. Against the interior of the steel wall is placed a ring of high strength castable refractory material 5. Against the castable refractory ring 5 and resting upon the firebrick wall 3, a ring 6 of specially shaped firebrick sections is placed. These sections are commonly called "skewback bricks". The hearths 7 are constructed of firebrick, in the general form of a circular, upwardly arched dome or conical frustum. In some cases, the roof 8 is also constructed as an upwardly arched dome or cone, in other cases it is a flat slab of castable refractory material, anchored to a flat steel top plate 9.
Because of the thickness of the wall section (commonly over 12 inches) and the insulating capacity of its inner layer 2, there will be a substantial temperature difference between the inner firebrick wall and the outer steel wall when the furnace is in operation. This temperature difference, plus the differences in expansion coefficients between the steel and the brick, results in an uneven expansion across the wall, with the brick side expanding much more than the steel side. The typical wall design includes only one expansion joint 10, between the top of the firebrick wall and the interior surface of the furnace roof 8. In recently built furnaces, it has been found that this design provision is inadequate to protect the furnace from damage due to differential expansion between the inside and outside portions of the wall. It is believed that this is due to the closer dimensional tolerances and flatter, smoother surfaces of modern firebrick as compared to the products of past years, which improvements are due to advancements in firing techniques and better quality control. An unforeseen result of using these bricks in the traditional design is that there are now far fewer "informal" sites where expansion can be taken up. Before, bricks often rested against each other on a few high spots, which spots quickly crumbled due to expansion pressure or wore off due to expansion-induced movement. Now, resting more solidly against each other, the effect of expansion of each brick is more nearly directly accumulative. Results of this inadequately controlled expansion can be serious.
The most obvious effect of inadequately controlled expansion are found on the hearths. One effect which has been observed is excessive rising of the hearths. Whenever the furnace is heated up, the bricks in the interior wall expand vertically. The furnace floor 11 attempts to restrain the bricks from moving downward. Since the skewback bricks are an integral part of the wall, the design assumes that they will also move upward, raising their hearths with them. It can be seen that each successive higher hearth will rise more than the next lower one. This amount of rise is added to the expected rise due to expansion of the hearth bricks themselves and is accounted for in the initial design by careful placement of doors and other interior projections above the expansion zone.
Problems arise, however, due to uncontrolled "locking in" of hearths. Referring to FIG. 2, it can be seen that because of its arched configuration and the special shape of the skewback, the hearth loads, live and dead, are transmitted into the skewback ring and transformed into a major horizontal thrust and a lesser downward vertical thrust. The vertical load is resisted by friction between the skewback 6 and the castable ring 5 and then by friction between the castable ring 5 and the steel wall 1. The horizontal load is resisted by ring tension in the buckstay band 4. It is not possible to accurately design the friction surfaces between 6 and 5 or 5 and 1. Therefore, it cannot be predicted with certainty that when the firebrick wall 3 expands due to heat, the skewback 6 will move upward relative to the castable 5, or the castable 5 will move upward relative to the steel wall.
If the skewback does not move upward during hot operation of the furnace, or if it wedges after rising and does not settle back when the furnace cools, the hearth is said to be "locked in". In some cases, "locked in" hearth will have a resistance which exceeds the compressive strength of the firebrick 3 comprising the interior wall. In this case the wall brick expansion forces will cause the bricks to crack or spall, causing permanent damage.
Another problem, caused by hearths "locked in" after rising, is that on cooling down, the brick below the locked hearth contracts and settles back down, leaving open gaps. Because of the turbulence and the heavy particulate loading of the furnace gases, small amounts of ash are blown into these gaps. On the next heatup, the full gap is no longer available to take up wall expansion, and more pressure is applied to the upper skewback. In some cases, the skewback resists, causing cracking and spalling due to overstress of the bricks. Other times, the skewback moves up to a new higher elevation, not contemplated in the original design and then locks at the new location. After a number of such cycles, the hearth rises far enough to interfere with other furnace internals, and the wall bricks above the rising hearth (or hearths) apply pressure to the internal wall projections such as burners and thermocouples, damaging them as well as the bricks.
Still another problem of prior art wall brick expansion is loosening and movement of the bricks in the hearth. FIG. 3 shows a situation where downward pressure due to a locked hearth above has caused the skewback brick 6 to pivot from its original position (as shown on FIG. 1) as a result of expansion of firebrick 3A and resistance of firebrick 3 comprising the interior wall. This causes some downward displacement of the hearth bricks 7a, 7b and 7c, which reduces the bearing surface between brick 7a and skewback 6, causing wear on the surfaces. It also produces localized overstress situations, causing cracking and spalling as shown in FIG. 3. After repeated heatup-cooldown cycles, the hearth bricks will slip down significantly, causing a measurable flattening of the arch, which reduces its structural stability. In some cases, because of the changed arch geometry, the hearth arch will no longer rise on heatup to compensate for hearth brick expansion. Instead, the hearth tends to grow larger in diameter. This overstresses the buckstay band and shell wall, causing them to stretch permanently. The hearth bricks will then drop still further on subsequent cooldown, and the wall and band stretching will increase on following heatups, until ultimately the hearth collapses.
The above are examples of the types of damage commonly observed in multiple hearth furnaces after several years of operation including 40 to 50 start-up-cooldown cycles. These are not the only modes or mechanisms for causing damage, but are selected because they can be more easily described than some of the others. Overall, uneven expansion across the multiple hearth furnace wall, as commonly designed during the past 60-plus years, is a serious contributor to shortened refractory life.